Retired U.S. Coast Guard Adm. Thad Allen said a weather system brewing in the Caribbean could force crews to abandon their watch over the experimental cap that’s been bottling oil a mile below the surface of the water for nearly a week.

Scientists have been scrutinizing underwater cameras and data for days, trying to determine if the cap is displacing pressure and causing leaks underground. If storms keep them from seeing the cap and getting those readings — for up to four days, Allen said — BP could reopen the well to avoid missing signs it is buckling.

“This is necessarily going to be a judgment call,” said Allen, the federal government’s point man on the Gulf of Mexico oil spill who will make the ultimate decision.

Forecasters say the storm system likely will move into the Gulf of Mexico over the weekend, although it appears to be weakening. Right now, it has a 50 percent chance of becoming a tropical depression or storm within the next 48 hours.

Even if the storm doesn’t hit the area directly, it could affect containment and cleanup. Hurricane Alex didn’t get closer than 500 miles from the spill in late June, yet offshore skimming in Alabama, Mississippi and Florida was essentially curtailed for nearly a week.

In Florida, crews were removing protective boom intended to buffer the state’s inland waterways in the Panhandle from oil. High winds and storm surge could carry the boom into sensitive wetlands, damaging those areas.

Allen said BP and government scientists were meeting to discuss whether the cap could be monitored from the shore.

It could take several days to evacuate ships from the well site 50 miles off the Louisiana coast, where the BP-leased Deepwater Horizon rig exploded April 20, killing 11 and touching off one of America’s worst environmental crises.

Allen said an evacuation could delay operations as much as two weeks before work would resume to kill the well at the bottom.

Shell Oil, the U.S. arm of Royal Dutch Shell PLC, already has begun evacuating personnel not essential to producing and drilling on their operations in the Gulf.

BP crews are in the final stages of readying a relief tunnel before boring into the side of the ruptured well to dump heavy mud and cement, sealing it for good. BP also may pump mud and cement from the top, to make efforts at the bottom easier. That procedure, called a surface kill, would occur before the well is ultimately plugged from below.

Before talk of nasty weather, BP was inching closer to completion and had hoped for a permanent plug by early August.

The temporary cork in the well has helped cleanup efforts, and Allen said skimming vessels are starting to have trouble finding oil to collect. BP has about 1,600 boats operating daily in waters off Alabama, Florida and Mississippi, 600 fewer than last week, said Matt Kissinger, director of BP’s “vessels of opportunity” program in the region.

Some boat captains, many earning more through the cleanup than they typically do from fishing, are worried it’s a sign BP is leaving the Gulf too early.

Shrimper Minh V. Le of Bayou La Batre had both of his boats out skimming for oil initially, but one has been deactivated.

“A lot of us have put a lot of sweat into the program,” he said. “You’ve got a 100-degree heat index, and there’s a lot of wear and tear on our boats. If something breaks down it can cost $30,000. What they’re paying isn’t a drop in the bucket.”